Though polycarbodiimide resins are thermosetting resins having excellent heat resistance, they are slightly soluble in various solvents and have low fluidity when melted, so that their mold processing becomes difficult in their practical use. With the aim of improving mold processing ability of such a polycarbodiimide, JP-A-51-61599, JP-A-61-246245, JP-A-4-261428 and the like disclose preparation of a powdery polycarbodiimide using 4,4′-diphenylmethane diisocyanate (MDI hereinafter) and a monoisocyanate. Though such a polycarbodiimide can be molded into a sheet shape by hot pressing, its workability cannot be said high and the moldings are hard and poor in flexibility.
Also, JP-A-2-292316 and JP-A-5-310877 describe a method for producing a polycarbodiimide solution using a specified solvent in carrying out polymerization from tolylene diisocyanate (TDI hereinafter). Though there are descriptions therein stating that the moldability is improved, flexibility of the hardened products is lower than that of the polycarbodiimide obtained from 4,4′-diphenylmethane diisocyanate.
In addition, JP-A-11-322888 and U.S. Pat. No. 5,357,021 disclose that flexibility of a polycarbodiimide is improved by introducing a long chain polyether or a long chain polyester into its molecule through urethane bond, urea bond or the like. However, heat resistance of the polycarbodiimide is sharply reduced by such a modification.